Add 2nd arg to signrawtransactionwithkey examples

Summary: The RPC examples for signrawtransactionwithkey are missing the 2nd parameter.

Test Plan:
./src/bitcoin-cli help signrawtransactionwithkey must end with following examples:

Examples:
> bitcoin-cli signrawtransactionwithkey "myhex" "[\"key1\",\"key2\"]"
> curl --user myusername --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "signrawtransactionwithkey", "params": ["myhex", "[\"key1\",\"key2\"]"] }' -H 'content-type: text/plain;' http://127.0.0.1:8332/

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7818